With an unconventional four-seat, twin-fuselage configuration, Pipistrel's winning Taurus G4 had a 75-ft. wing span and a 150-kw electric motor. “We did not settle for 200 passenger-mi./gal. and wanted as much as possible,” says Tomazic. “On paper it would do over 500 passenger-mi./gal., and in testing it achieved that, flying in a straight line.”
“The first moment I saw the Pipistrel entry on paper, my reaction was 'That's exactly what I would have done',” says Moore. “To me, their effort showcased exactly the intent: Push aerodynamic efficiency as hard as you can, while realizing that you only have a year to design, develop, build and test-fly a custom one-of-a-kind aircraft.”
NASA was barred from awarding prize money to non-U.S. teams, so Pipistrel partnered with Jack Langelaan, associate professor of aerospace engineering at Pennsylvania State University. “Back-of-the-envelope calculations showed that a modern self-launching sailplane would exceed the minimum criteria by a significant margin, and that the Pipistrel Taurus was a great candidate for modification to compete,” he says.
Langelaan and Tomazic met at a convention in spring 2010. “Because the competition was limited to U.S. teams, they needed me as much as I needed them,” Langelaan says. “And because my group had been doing research on flight planning and control to maximize efficiency, I was able to make a technical contribution.”
The Taurus G4 won, achieving 403.5 passenger-mi./gal. at 107 mph—more than double the threshold. But did winning the $1.35 million prize pay off for Pipistrel? The previous PAV and GAT challenges clearly advanced the company's product plans—it has delivered more than 170 Virus aircraft since 2009 and it remains a best-seller, Tomazic says, but the unique G4 was not practical because of its configuration and size.
“We learned a lot,” he says. The G4 had the largest battery pack ever flown, 10 times larger than the Boeing 787's. “We learned how to manage such a large battery, prevent discharge, handle thermal management and keep everything in balance.” Pipistrel also learned to manage electrical grounding and electromagnetic interference in an all-electric, all-composite aircraft where the largest piece of metal was a 7-lb. engine mount.
Since Green Flight, Penn State has “seen a big jump in interest for electric and hybrid aircraft,” says Langelaan. “Many students in our capstone aircraft design class are working on electric-powered general aviation aircraft . . . [and] we're starting plans to convert a sailplane to an electric aircraft as part of the design course.”
“Thanks to both Pipistrel and the [second-place] University of Stuttgart, breakthrough technologies for efficiency, emissions and noise were showcased, and I believe we achieved an epiphany concerning electric flight,” says Moore. “There is so much more momentum relating to electric flight than two years ago.”
Despite the G4's uniqueness, competing in Green Flight was “directly useful,” says Tomazic. Elements of the design have found their way into the latest version of Pipistrel's two-seat self-launching glider. Although it has a 45-kw motor, and not the G4's 150 kw, “the battery-management electronics are identical,” he says. “Some lessons we will use later, some we will avoid in the future, and that is valuable, too.”